Currently, the surface of the existing insulating paper is not attached with any insulating paint, so that after covering a conductor, the insulating paper becomes damp in the process of winding a transformer coil, which results in reduction of the electrical performance. Therefore, the insulating paper is dried after winding the coil, and then, the coil is immersed into the insulating paint covering the surface of the insulating paper after drying in order to prevent the insulating paper from becoming damp, so as to ensure the electric performance is not reduced. But if relatively more numbers of layers of insulating paper cover the conductor, it is not close enough between layers and the insulating paint fails to completely immerse into each layer of insulating paper.